Cost-effective treatment of per- and polyfluoroalkyl substance (PFAS)-impacted matrices, such as groundwater, would be facilitated if a high-efficiency, high-capacity sorbent that is regenerable on-site, for re-use could be developed, while producing an aqueous concentrate compatible with a practical treatment for complete PFAS destruction. Preliminary data had indicated that ionomers, a class of novel and water-swellable sorbents, might have the required characteristics and can be combined with hydrothermal alkaline treatment (HALT) for PFAS mineralization. This proof-of-concept project was designed to determine ionomer’s PFAS sorption kinetics and capacity relative to commercial sorbents, and to evaluate the desorption and destruction of PFAS in the resulting concentrate.

The investigated ionomers contain a high density of cationic amine groups distributed along a cross-linked polymer backbone that controls their swelling. A single ionomer formula was used for most of the testing in this project, while other formulas with different extents of cross-linkage may also be tailor-designed for specific applications. Batch and column sorption and desorption kinetic and equilibrium tests were coupled with HALT experiments using aqueous solutions containing single or multiple model PFAS. Sorption performance of ionomer was compared to those of commercial PFAS sorbents, such as granular activated carbon (GAC) and ion exchange resin (IX), and with literature data for other novel sorbents. Modeling results, supported by short column tests, suggest that repeated cycles of sorption-regeneration operations would be feasible.

Sorption kinetic tests, using perfluorooctanoic acid (PFOA) and a mixture of perfluorobutanoic acid, perfluorobutanesulfonic acid, PFOA and perfluorooctanesulfonic acid (PFOS), showed that sorption to the ionomer was significantly faster than to GAC and IX. Sorption of PFOA to ionomer followed a Langmuir isotherm, with a maximum sorption capacity greater than those reported for the other sorbents. Tests using a PFAS mixture showed the sorption capacity of the ionomer exceeded that of GAC for all four PFAS and that of an IX resin for PFOA and PFOS. A novel dilute (<1%, w/w) aqueous regenerant was found to be effective at desorbing and recovering the four model PFAS from used ionomer, producing an aqueous concentrate that was conducive to PFAS destruction by HALT. Modeling based on these sorption and desorption data supports repeated sorption-regeneration cycles at high efficiency, with a concentration factor potentially exceeding 1,000. A series of five-cycle column sorption-regeneration tests showed that the ionomer could be re-used effectively after desorption. HALT was applied directly to treat mixed PFAS concentrates and achieved high degrees of mineralization (ca. 80 to >95% fluoride recovery) under subcritical conditions. Higher temperatures were required to achieve extensive defluorination than to destroy the parent PFAS, suggesting the formation of a recalcitrant fluorinated intermediate(s).

The results support the use of ionomers for sorption, on-site desorption, and destruction of PFAS in impacted waters. The proposed treatment can take place entirely in water without the use of alcohol. Additional studies are needed to clarify the conditions under which the ioners are most applicable. Further studies under SERDP are not being conducted at this time. (Project Completion - 2024)